Effect of grain size and residual stresses on R-curve behaviour of alumina based composites

Alumina samples with average grain sizes ranging from 2.6 to 67.4 μm and alumina based composites containing 10–30 wt.% of second phase particles with lower (SiC) and higher thermal expansion coefficient (ZrO2+8 mol% Y2O3) were prepared. The effect of grain size and residual stresses on R-curve behaviour of ceramics was investigated by testing controlled crack growth during three point bending of single-edge-notched samples. The tests showed that R-curve behaviour of alumina ceramics strongly decreases with the decrease of Al2O3 grain size. Observed changes were related to the amount of bridging grains on the path of the propagating crack and residual stresses present in ceramics. The same tests done on composite samples showed that ZrO2 and SiC addition also strongly decrease the effect of R-curve in alumina matrix. This phenomenon was explained by analysis of Al2O3 grain size, amount of bridging grains and residual stresses in ceramics.